2 Log of characteristics of observational data

Observations of the nuclear region of the Seyfert galaxy NGC 4151 have been carried out at the Crimean Astrophysical Observatory with the 1.25 m telescope using an aperture 20 arcsec in diameter. Over 96 nights, 8 observational runs were made between February 11, 1989 and June 14, 1996. We performed more than 1500 measurements in each of 5 spectral bands of Johnson's UBVRI system simultaneously. One measurement consists of 8 integrations of 10 s each. Photon statistics corrected for sky background are applied to calculate photometric errors, which were taken to be the same as the rms errors obtained by averaging eight integrations. Taking into account the noise variations of sky background and of a comparison star, we derived the following rms errors for a single observation in the UBVRI bands, respectively: 0 $.\!\!^{\rm m}$011, 0 $.\!\!^{\rm m}$006, 0 $.\!\!^{\rm m}$007, 0 $.\!\!^{\rm m}$007 and 0 $.\!\!^{\rm m}$004. About 50% of observational nights lasted more than 1 hour; the longest intranight observational run lasted 7.3 hours. The circumstances of the observations were discussed in detail by Merkulova et al. (1999a,b).

The light curves of NGC 4151 in each of the UBVRI bands consist of a slow increase of intensity over almost all the observing time, with 10-150 day outbursts superimposed on it (see Fig. 1). The increase of the nucleus brightness began in February, 1990. The extraordinary long-term brightening continued until June, 1996. The amplitudes of brightening and gradients decreased from the U to the I band. The amplitudes amounted to 2 $.\!\!^{\rm m}$2, 1 $.\!\!^{\rm m}$5, 0 $.\!\!^{\rm m}$9, 0 $.\!\!^{\rm m}$8 and 0 $.\!\!^{\rm m}$7 in the UBVRI bands, respectively. The variability in the I band was at the level of about 10% per year. Observations obtained in 1997-98 with Hubble Space Telescope using a technique of high sensitivity and resolution of near-infrared imaging detected variability of the NGC 4151 nucleus at 1.6 $\mu$m at the same level: 11% over 7.3 months (Quillen et al. 2000). The amplitudes of the UBVRI flux variability $F_{\max}$/$F_{\min}$ for ten day flares were 3.4, 1.8, 1.4, 1.4 and 1.3, respectively. The amplitudes of intranight variations were as much as 15%. The amplitudes of all levels of flux variability were significantly higher than the observational errors.

Figure 1: The light curve of NGC 4151 obtained in the spectral band V using a circular diaphragm 20'' in diameter. The circles are nightly mean values of flux densities in mJy plotted versus the Julian Dates. Bars are rms variations about mean values. Dashed line shows a low boundary of the light curve. Four periods of observations under consideration are indicated by roman numerals.

A lot of the observational data permits us to search for the characteristics of the flux of the NGC 4151 nucleus variability with time, dividing all observational periods into four parts with a duration of 1.5-2.0 years as indicated in Table 1 and in Fig. 1. The duration of each period of time was defined approximately.

The columns of this table show: (1) the number of periods of observation; (2) the starting and ending Julian Dates of the period of observations (JD = jd + 2400000 days); (3) the duration of the observations dt, days; (4) the number of observational nights (n); (5) the number of measurements (n₁); (6) the power law spectral slopes of flux excesses for I-IV periods of the nucleus brightening ( $\alpha _{\rm pl}$) according to Merkulova et al. (1999a).

 

 

Table 1: Characteristics of four observational periods.

Period	jd	dt,	n,	n1,	$\alpha _{\rm pl}$
of observ.	days	days	nights	points
1	2	3	4	5	6
I	47569-48216	648	18	190	-1.17
II	48216-48764	528	22	360	-1.21
III	48764-49479	522	10	167	-0.90
IV	49479-50249	764	46	841	-0.37